Electronic circuits using small signal transistors and integrated circuits are easily damaged by high voltage transients that may appear on the line due to lightning, switching transients, or power line induction. Accordingly some form of transient and over-voltage protection must be used to protect the solid state circuits contained in Central Office equipment of local telephone companies or in the so-called "station" equipment at a customer's premises.
Since technology used for making integrated circuits contained in this equipment cannot provide high enough breakdown voltage, other protection devices are employed. For example, one or more external zener diodes are placed across the line to provide over-voltage protection. When the input voltage exceeds the zener (breakdown) voltage, the zener conducts and holds the voltage input to the electronic circuit at the rated zener voltage. In other prior art devices, metal oxide varistors are connected from the protected line to ground on another line. Still other prior art devices employ ionization of a gas between two electrodes to provide a voltage breakdown device. Prior art protection against abnormal currents employ such mechanical devices as heat coils or fuses. Also standard enclosures have been developed in which such prior devices are mounted during use.
While such prior art devices provide results in the area intended, there still exists a great need for a new and improved protector unit which employs existing components, and maximizes operating range and minimized cost. Additionally, it is important that solid state protector units be constructed to mount easily into standard enclosures which have been developed for used in Central Offices and so-called "Building Entrance Terminals", which are multi-line (12-,25-,50-,100-,and 200- lines, for example) equipment boxes installed for business and apartment buildings.
Accordingly, a principal object of the present invention is to provide a compact solid state protector unit for use as a protector for solid state Central Office and multi-line telephone station equipment.
Another object of the present invention is to provide a compact solid state protector unit which is constructed and arranged to be mounted in standard Central Office and Building Entrance Terminal equipment jacks.
A still further object of the present invention is to provide a compact solid state protector unit having access means for testing the electrical characteristics of the telephone lines.
A still further object of the present invention is to provide a compact solid state protector unit which is constructed and arranged to permit partial disconnection of the premises equipment from the telephone loop itself.
Other objects and advantages of the invention will in part appear hereinafter and will in part become apparent after consideration of the specification with reference to the accompanying drawings.